The physics
A fork rings over a tube of water. The water closes the tube below. So the air column is a closed pipe.
The column booms when its own note matches the fork. That is resonance. The water surface is a displacement node. The open top is an antinode.
l₁ = λ / 4 l₂ = 3λ / 4 next booms: λ / 2 apart
The air swings a little above the open top. So each boom length is short by the end correction e. It is about 0.3 times the tube's width.
l₁ + e = λ / 4 v = 2f(l₂ − l₁) e = (l₂ − 3l₁) / 2
Dip an open pipe into water. The air above the water becomes a closed pipe. Half dipped, it sounds the same lowest note as before.
dipped: f = v / 4(L − depth)